1. Field of the Invention
The present invention relates to a reduced-pressure drying unit and a coating film forming method for vaporizing a solvent from a coating solution when the coating solution made by, for example, mixing a component of a coating film in the solvent, for example, a resist solution is applied to a substrate such as a semiconductor wafer or an LCD substrate (a glass substrate for a liquid crystal display) to form the coating film.
2. Description of the Related Art
In fabrication processes of semiconductor devices and LCDs, resist processing is performed for a substrate to be processed by a technique called photolithography. This technique includes a series of steps of applying a resist solution to, for example, a semiconductor wafer (hereinafter, referred to as a wafer) to form a solution film on its front face, exposing the resist film to light using a photomask, and thereafter performing a developing treatment to obtain a desired pattern.
There is one method for the above-described coating treatment of the resist solution, in which, for example, as shown in FIG. 30, a coating solution is applied in a manner of so-called drawing with one stroke such that the coating solution (the resist solution) made by mixing a resist that is a component of a coating film and a solvent is discharged onto a wafer front face from a nozzle 10 while the nozzle 10 provided above the wafer W is being reciprocated in an X-direction and the wafer W is intermittently moved in a Y-direction. Numeral 12 in FIG. 30 denotes a mask which covers a region other than a circuit formation region 11 of the wafer.
For carrying out the above-described method, it is considered to be preferable that the wafer is carried into a reduced-pressure drying unit for reduced-pressure drying immediately after the coating solution is applied onto the wafer because a solvent having a low vaporization is used as the solvent contained in the coating solution and the uniformity of the film thickness of the coating film is secured by quickly removing the solvent from the front face of the wafer.
FIG. 31 is a view showing a conventional reduced-pressure drying unit in which numeral 13 denotes a hermetic container constituted by a lid body 14 and a mount 15, and a ceiling portion of the lid body 14 is formed with an opening 14a. The opening 14a can communicate with a vacuum pump 16 through an exhaust pipe 14b to reduce the pressure in the hermetic container 13. In such a unit, the wafer W is mounted on the mount 15 and heated by a not shown heater, and the vacuum pump 16 is actuated to reduce the pressure in the hermetic container 13, thereby causing the solvent, for example, a thinner or the like remaining on the front face of the wafer W to vaporize (dry) and the vaporized solvent to be sucked toward the vacuum pump 16 side, so that the resist component in the coating solution remains on the front face of the wafer W.
Meanwhile, a coating solution 17 on the front face of the wafer, when carried into the reduced-pressure drying unit, is rounded, for example, as shown in FIG. 32, at a peripheral region of the wafer (a region inside by a predetermined distance, for example, about 20 mm from the periphery) due to a surface tension of the coasting solution itself. Therefore, it is considered to provide a current plate 18 above the wafer W mounted on the mount 15 in such a manner that the current plate 18 faces the wafer W as shown by a dotted line in FIG. 31. When the current plate 18 is provided as described above and the pressure in the hermetic container 13 is reduced, an air current is formed which spreads outward between the current plate 18 and the wafer W to cause the coating solution to spread toward the periphery on the front face of the wafer.
Therefore, if the current plate 18 is provided and reduced-pressure drying processing is performed, the coating solution is drawn toward the periphery as shown in FIG. 33, resulting in an extremely large thickness of the coating film at the peripheral region. The coating film with a rounded shape or an accumulated portion at the peripheral region as described above cannot be used as a circuit formation region because the peripheral region of the coating film is greatly different in film thickness from the center region. Hence, there is a request that the circuit formation region occupies an area as large as possible to improve the yield of chips per wafer, and thus a significant problem is a control of the film thicknesses at the center region and the peripheral region in the coating film formed on the front face of the wafer.